The presence of transients in video signals can give rise to distorted pictures. In practice, it is found that to the human eye, a viewer watching a television will be far less disturbed by transient signals that have symmetrical waveforms, rather than those with asymmetrical wave forms. Asymmetrical transient signals can be caused, for instance, by reflections within coaxial cables, by incorrect transcoding of channels in cable systems and by various other factors.
FIGS. 1A to 1C illustrate symmetrical transient signals. FIG. 1A shows an ideal step response, FIG. 1B a step response with symmetrical overshoots, and FIG. 1C a symmetrical but bandwidth limited step response.
FIGS. 2A and 2B show asymmetrical step responses that are asymmetrically distorted versions of the step responses of the FIGS. 1B and 1C signals. The distortion shown in FIGS. 2A and 2B generally occurs due to transmission errors or problematical signal processing which causes distortion of the symmetrical transients of FIG. 1. FIG. 2B shows an example of a pre-shoot and FIG. 2A an after-shoot. In these Figs., the ideal response is shown dashed (- - - ). FIG. 2A shows a response in which the ideal is deviated from to a greater extent upon reaching V2 by over shooting V2 before settling back down. In FIG. 2B, the actual response deviates from the ideal response by dipping below V1 to a relatively large extent prior to the step change between V1 and V2 (this may be regarded as a form of “pre-shoot”).
Other types of asymmetrical step responses can be envisaged, and may be caused by e.g. clipping.
U.S. Pat. No. 4,404,584 (RCA) describes an apparatus in which asymmetrical peaking is carried out, but this apparatus does not provide any means for detecting asymmetry in transient signals.
U.S. Pat. No. 5,339,115 (THOMSON) describes a symmetry control circuit with variable delay being imposed on signals according to a selected signal source.
JP-A-02230872 (MAKOTO MIYAHARA) discloses a phase distortion correction in image pickup tubes in which correction is achieved by distorting the input signal with an inverse characteristic.